This invention relates to fibers. In one aspect, the invention relates to fibers comprising a polyolefin polymer and a photoinitiator while in another aspect, the invention relates to such fibers crosslinked through the action of ultraviolet (UV) radiation. Other aspects of the invention include a method of making the fiber, and structures made from the fibers.
Fibers with excellent elasticity are needed to manufacture a variety of fabrics which are used, in turn, to manufacture a variety of durable articles such as, for example, sport apparel and furniture upholstery. Elasticity is a performance attribute, and it is one measure of the ability of a fabric to conform to the body of a wearer or to the frame of an item. Preferably, the fabric will maintain its conforming fit during repeated use, extensions and retractions at body and other elevated temperatures (such as those experienced during the washing and drying of the fabric).
Fibers are typically characterized as elastic if they have a high percent elastic recovery (that is, a low percent permanent set) after application of a biasing force. Ideally, elastic materials are characterized by a combination of three important properties: (i) a low percent permanent set, (ii) a low stress or load at strain, and (iii) a low percent stress or load relaxation. In other words, elastic materials are characterized as having the following properties (i) a low stress or load requirement to stretch the material, (ii) no or low relaxing of the stress or unloading once the material is stretched, and (iii) complete or high recovery to original dimensions after the stretching, biasing or straining is discontinued.
Spandex is a segmented polyurethane elastic material known to exhibit nearly ideal elastic properties. However, spandex is cost prohibitive for many applications. Also, spandex exhibits poor environmental resistance to ozone, chlorine and high temperature, especially in the presence of moisture. Such properties, particularly the lack of resistance to chlorine, causes spandex to pose distinct disadvantages in apparel applications, such as swimwear and in white garments that are desirably laundered in the presence of chlorine bleach.
Elastic materials comprising polyolefins, including homogeneously branched linear or substantially linear ethylene/xcex1-olefin interpolymers, are known, e.g., U.S. Pat. Nos. 5,272,236, 5,278,272, 5,322,728, 5,380,810, 5,472,775, 5,645,542, 6,140,442 and 6,225,243. These materials are also know to exhibit good resistance to ozone, chlorine and high temperature, especially in the presence of moisture. However, polyolefin polymer materials are also know to shrink upon exposure to elevated temperatures, i.e., temperatures in excess of ambient or room temperature.
The concept of crosslinking polyethylene with UV-light in the presence of a photoinitiator was first published in the 1956. After this, the photocrosslinking of polyolefins became the subject of a substantial amount of research. The majority of this work focused on the crosslinking of polyethylene using as the photoinitiator an aromatic ketone in which the carbonyl group is linked to two aromatic groups. Upon UV-irradiation, the carbonyl group is excited into a triplet state in which it can abstract a hydrogen atom from a suitable donor.
EP 0 490 854 B1 describes a process and equipment for the continuous crosslinking of polymeric materials, in particular polyethylene, using UV-light and both a photoinitiator and a crosslinker. The photoinitiator is a benzophenone derivative with high molecular weight and low vapor pressure and a representative crosslinker is triallyl cyanurate.
U.S. Pat. No. 4,190,602 and Ger. Offen. 2,337,813 describe UV-curing of polyester resins using monoacetals of aromatic 1,2 diketones. In one example, photocrosslinking of polyethylene with Irgacure 651 (a monoacetal of aromatic 1,2 diketone) is described. In that procedure, the Irgacure 651 was worked into polyethylene of density 0.92 using mixing rolls, and the resulting rolled sheet was compression molded into 0.1 mm thick film. This film was irradiated for 40 minutes with a high-pressure mercury lamp and extracted in boiling toluene for 5 hours to give 24% of insoluble material. This example was limited to film structures.
As suggested above, much of the reported literature on the photocrosslinking of a polyolefin is limited to the photocrosslinking of a polyethylene or polypropylene resin. In many reported examples, both a photoinitiator and a photocrosslinker are required for efficient photocrosslinking. This is particularly the case with benzophenone and its derivatives, which often required a multifunctional photocrosslinker.
According to one embodiment of this invention, a temperature-stable, elastic, polyolefin polymer filament fiber substantially crosslinked solely as a result of a photoinitiator activated by exposure to UV-radiation, i.e., without the use of a photocrosslinker, is described. In another embodiment, the photoinitiator is an aromatic ketone, e.g., benzophenone, anthrone or one of their derivatives. In another embodiment, the polyolefin polymer fiber contains both a photoinitiator and a photocrosslinker, preferably a multifunctional photocrosslinker, e.g., a triallylisocyanurate.
In another embodiment of the invention a temperature-stable, substantially crosslinked, elastic fiber is prepared by a process comprising the steps of (a) providing a mixture of a polyolefin polymer and a photoinitiator; (b) forming the mixture into a fiber; and (c) exposing the fiber to UV-light for a duration sufficient to cause the fiber to have a gel level of at least 30 weight percent, as determined by ASTM D-2765. In a variation on this embodiment, the mixture includes a photocrosslinker. In another variation on this embodiment, the fiber is first formed from the polyolefin polymer, and then the photoinitiator and/or photocrosslinker is applied to the fiber.
In another embodiment of this invention, the temperature-stable, substantially crosslinked, polyolefin polymer elastic fiber is used to make a yarn, either covered or uncovered. Other embodiments of the invention include woven, nonwoven and knitted fabrics comprising one or more of the elastic fibers or yarn of the invention, and elastic structures or articles comprising one or more of the fibers of the invention.